(1) Field of the Invention
The invention relates to the manufacturing of high performance Integrated Circuit (IC's), and, more specifically, to a wafer structure and fabrication method to form both gold and solder bumps on the same wafer.
(2) Description of the Related Art
Integrated circuit (IC) packaging techniques must be able to accommodate the developing trends of electronic devices, including digitization, networking, localization, and user friendliness. In order to meet these requirements, electronic devices must be functional, highly integrated, miniaturized, lightweight, inexpensive, and high speed. There is a soaring demand for incorporating many more functions on one chip while still having few bonding pads to link the chip to other substrates. High-density IC packages such as ball grid arrays (BGA), chip scale packages (CSP), flip chips, and multi-chip modules (MCM) have been developed. For high-density IC packaging, the speed of signal transmission increases as the wiring length decreases. Thus, the application of bumps has become a major trend in high-density packaging.
Having solder bonding and wirebonding capabilities simultaneously on a same wafer is desirable for applications of chip-on-chip or chip-on-silicon-substrate assembly. Currently, solder bonding is provided by creating solder bumps on a subset of the bonding pads, while wirebonding is performed on the remainder of the original bonding pads; that is, on the aluminum surface. However, the process of creating solder bumps on the wafer will often damage the aluminum bonding pads, making it impossible to perform wirebonding on the original aluminum bonding pads.
In our invention, we create gold bumps or gold pads to protect the aluminum pads before performing the solder bumping process. The gold bumps or gold pads can be used for the wirebonding or TAB (Tape-Automated-Bonding). Having both solder bumps and gold bumps placed on one wafer enables the possibility of chip-on-chip or chip-on-silicon-substrate packages. A process to form both solder bumps and gold bumps on the same wafer will allow a large chip to be solder bonded face-to-face with one or several smaller chips and later wirebonded or TAB (Tape-Automated-Bonding) to the next layer of an assembly (for example, organic, ceramic or glass substrate).
U.S. Pat. No. 6,492,692 to Ishii et al describes solder and gold bumps together, but gives no details about how these two types of bumps are to be formed on a same wafer. A barrier metal layer of TiW and Au is disclosed for gold bumps. U.S. Pat. No. 6,228,689 (Liu), U.S. Pat. No. 5,655,639 (Seppala et al), and U.S. Pat. No. 5,532,612 (Liang) teach diffusion/barrier layers of TiW and Au under gold bumps. The last patent teaches that formation of solder and gold bumps differs in many details. U.S. Pat. No. 6,383,916 to Lin et al, assigned to the same assignee as the present invention, adds, in a post passivation processing sequence, a thick layer of dielectric over a layer of passivation and layers of wide and thick metal lines on top of the thick layer of dielectric.